Generalized spike-and-wave (SW) seizures comprise a major category of childhood-onset epilepsy. Neither the underlying basic excitability mechanisms nor the effects of early SW seizures on developing neural circuitry have been clearly defined. We propose to investigate the temporal onset of two cellular excitability defects favoring synchronization in the mouse mutant stargazer (stg), a genetic model of SW epilepsy. We hypothesize that: (1) the accelerated burst frequency in stg hippocampal CA3 pyramidal neurons is a primary network defect resulting from aberrant membrane repolarization, and (2) the unique pattern of dentate granule cell-mossy fiber axon terminal hyperplasia in the stg mutant brain is a secondary consequence of generalized SW epilepsy. Using intracellular recordings, we will test specific hypotheses regarding (1) membrane and synaptic repolarization defects in the mutant network, (2) their functional role in epileptogenesis, and (3) whether they arise as a primary expression of the stg mutant locus during early brain development or secondarily as a product of seizure-induced neuroplasticity. In specific Aims 1 and 2, we will pinpoint the birthdate of EEG seizures and excitability defects in early postnatal development in mutant and co-isogenic control neurons. In Specific Aim 3, we will examine the development of mossy fiber hyperplasia by combining zinc staining patterns with dye injections and compare the timing of synaptic reorganization to the onset of excitability defects identified in Aims 1 and 2. In Specific Aim 4, we will use receptor blockers and microdissection to isolate CA3 neurons from mossy fiber synaptic inputs. In Specific Aim 5, we will test whether aberrant axon growth is (1) a cause or an effect of the seizures, (2) mediated by excitotoxic cell death, and (3) reversible once seizures are controlled. These studies will define specific epileptogenic cellular defects in the stg mutant and provide new information about basic mechanisms of SW seizures, the role of axon terminal sprouting and neosynaptogenesis in generalized epilepsy, and the mechanism and reversibility of long-term cellular neuroplasticity in hippocampal circuitry that may accompany childhood SW epilepsy.